The invention relates to sensing devices, and more particularly to devices for sensing changes in the velocity of a moving body.
There are many applications in science and industry for apparatus to measure velocity accurately and particularly to sense changes in velocity. Such velocity could be peripheral i.e. tangential or rectilinear.
One illustrative application in which such sensors could find great utility is in the aviation industry, and more particularly in the field of helicopter, or rotary wing, aviation. For the purposes of illustration, the following discussion will refer to the field of helicopter aviation but it should be noted that the invention is not limited to this particular field and that it may be employed in any industry or field of technology wherein it is desired to sense changes in the velocity of a moving body.
It has previously been proposed to employ sensors to monitor various components of rotary wing aircraft to enable mechanical problems to be spotted and dealt with before they pose a threat to the safety of the aircraft.
A number of recent air accidents have shown that the powertrain, and more particularly the geartrain, of an aircraft is particularly vulnerable to mechanical problems. To counter this threat, it has previously been proposed to install sensors to monitor the condition of the powertrain and gearboxes in particular. One such sensor which has previously been employed comprises an inertia device, such as a piezoelectric sensor, which is mounted on the geartrain housing, for example, of the aircraft.
It was previously postulated that an inertia device, such as a piezoelectric sensor, would be suitable for sensing the degradation of the gears within the housing, as any such degradation would give rise to vibrations that would cause the piezoelectric device to be subjected to inertia forces whereupon a signal would be generated that could be monitored. However, it has been noted by the present inventor that these inertial devices do not provide an accurate indication of the state of the gears within the housing. As a result of this, it is not uncommon for engineers to strip a gearbox, for example, in response to a detected problem only to find that the geartrain is in fact operating properly. In other instances, geartrains have failed without the sensors indicating that there is a problem to be addressed. It is apparent therefore that inertial sensors are not suitable when used to infer from a detected vibration the integrity of rotating, or otherwise moving, members of an aircraft component.
A number of alternative sensors have previously been proposed. For example, U.S. Pat. No. 4,751,459, European Patent Specification No. 0769700 and United Kingdom Specification No. 2257527 each disclose a sensing device comprising substantially U-shaped magnetic yoke which is fittable over the edge of a rotatable body to provide an indication of changes in the rotation velocity of the body. Whilst these sensing devices adequately enable the velocity changes of a moving body to be detected, they have their own set of associated problems and as such are generally unsuitable for use in certain applications.
For example, practical investigations have determined a number of problems associated with the use of these previously proposed sensors in the illustrative field of rotary wing aviation. A first problem is associated with the fact that the above mentioned systems are inordinately massive. In any device employed in an aircraft it is a requirement that the mass (weight) of the component should be reduced consistent with satisfactory formation.
A second problem is associated with the fact that all of the previously proposed mechanisms necessitate the provision of a magnetic circuit that extends from one face of the rotating component around the periphery of the component to the other face thereof. This is disadvantageous as the extension of the sensor beyond the periphery of the component requires the component casing to be enlarged in order to accommodate the sensor. It has also been noted that the portion of the magnetic circuit extending around the periphery of the sensor represents a large proportion of the total weight of the sensor, and thus that the weight of the sensor could be significantly reduced if this portion of the magnetic circuit could be removed.